This invention relates to a method of preparing in a single step, a mixture comprising a p-brominated phenol and a p-benzoquinone. More particularly the method relates to a method of preparing in a single step, a mixture of p-bromophenol and p-benzoquinone intermediates which may be separated and subsequently converted in single step transformations to dihydroxy aromatic compounds useful in the preparation of polycarbonate copolymers.
A variety of copolymers possessing useful and desirable properties comprise structural units derived from both hydroquinones and 4,4xe2x80x2dihydroxybiphenyls. Examples of include the polyether sulfone (CAS No. 90337-94-3) prepared from hydroquinone (HQ), 4,4xe2x80x2-dihydroxybiphenol (BP) and bis(4-chlorophenyl)sulfone; and the polyester (CAS No. 96892-06-7) derived from HQ, BP and a mixture of iso- and terephthalic acid. Additionally, polycarbonates comprising structural units derived from HQ, BP and another bisphenol comonomer (e.g. CAS No. 491588-47-7) show promise in a variety of materials applications.
Typically, the hydroquinone derivative and the 4,4xe2x80x2-dihydroxybiphenyl derivative used in the preparation of such polymers are prepared in independent manufacturing steps. Hydroquinone is typically prepared by air oxidation and fragmentation of 1,4-diisopropylbenzene, or by direct oxidation of phenol. Typically, 4,4xe2x80x2-dihydroxybiphenyl is obtained by oxidative coupling of 2,6-di-tert-butylphenol followed by acid mediated removal of the tert-butyl groups in the coupled product. Hydroquinones may be prepared as well by hydrolysis of a p-bromophenol as illustrated in U.S. Pat. No. 1,934,656. In addition, 4,4xe2x80x2-dihydroxybiphenyls may be prepared by reductive coupling of a p-bromophenol to the corresponding 4,4xe2x80x2-dihydroxybiphenyl as described in U.S. Pat. No. 5,177,258. Co-pending U.S. application Ser. No. 10/342,475 (filed Jan. 16, 2003) discloses an efficient means of preparing a p-bromophenol as a single intermediate which can be transformed by hydrolysis or reductive coupling into either a hydroquinone or a 4,4xe2x80x2-dihydroxybiphenyl. While this approach provides additional efficiencies based upon its use of a single intermediate p-bromophenol relative to known methods, improved methods continue to be sought, especially in light of the challenges presented by the rigorous conditions required for the hydrolytic transformation of the intermediate p-bromophenol to the corresponding hydroquinone.
The present invention is related to that described in co-pending U.S. application Ser. No. 10/342,475 (filed Jan. 16, 2003), but provides an alternate approach to the preparation of hydroquinones and 4,4xe2x80x2-dihydroxybiphenyls which eliminates the need for hydrolytic conversion of a p-bromophenol intermediate into the corresponding hydroquinone.
In one aspect, the present invention provides a method preparing a mixture of a p-bromophenol and a p-benzoquinone, said method comprising contacting in a reaction mixture a hydroxyaromatic compound with:
(a) hydrogen bromide;
(b) at least one source of copper selected from the group consisting of copper compounds, and elemental copper; and
(c) oxygen gas;
said hydrogen bromide being present in an amount corresponding to between about 0.01 and about 0.2 moles of hydrogen bromide per mole of said hydroxyaromatic compound, said contacting taking place at a temperature in a range between about 20xc2x0 C. and about 250xc2x0 C.
In another aspect, the present invention relates to a method for the preparation of hydroquinones and 4,4xe2x80x2-dihydroxybiphenyls, said method comprising conversion of a mixture of a p-bromophenol and a p-benzoquinone into purified forms of the corresponding 4,4xe2x80x2-dihydroxybiphenyl derivative and the corresponding hydroquinone derivative.
The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included herein. In this specification and in the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings.
The singular forms xe2x80x9caxe2x80x9d, xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise.
xe2x80x9cOptionalxe2x80x9d or xe2x80x9coptionallyxe2x80x9d means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
As used herein the term xe2x80x9cpolycarbonatexe2x80x9d refers to polycarbonates incorporating structural units derived from one or more dihydroxy aromatic compounds and includes copolycarbonates and polyester carbonates.
As used herein, the term xe2x80x9cmelt polycarbonatexe2x80x9d refers to a polycarbonate made by the transesterification of at least one diaryl carbonate with at least one dihydroxy aromatic compound.
xe2x80x9cBPAxe2x80x9d is herein defined as bisphenol A and is also known as 2,2-bis(4-hydroxyphenyl)propane, 4,4xe2x80x2-isopropylidenediphenol and p,p-BPA.
As used herein the term xe2x80x9caromatic radicalxe2x80x9d refers to a radical having a valence of at least one and comprising at least one aromatic ring. Examples of aromatic radicals include phenyl, pyridyl, furanyl, thienyl, naphthyl, phenylene, and biphenyl. The term includes groups containing both aromatic and aliphatic components, for example a benzyl group, a phenethyl group or a naphthylmethyl group. The term also includes groups comprising both aromatic and cycloaliphatic groups for example 4-cyclopropylphenyl and 1,2,3,4-tetrahydronaphthalen-1-yl.
As used herein the term xe2x80x9caliphatic radicalxe2x80x9d refers to a radical having a valence of at least one and consisting of a linear or branched array of atoms which is not cyclic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of aliphatic radicals include methyl, methylene, ethyl, ethylene, hexyl, hexamethylene and the like.
As used herein the term xe2x80x9ccycloaliphatic radicalxe2x80x9d refers to a radical having a valance of at least one and comprising an array of atoms which is cyclic but which is not aromatic, and which does not further comprise an aromatic ring. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of cycloaliphatic radicals include cyclopropyl, cyclopentyl cyclohexyl, 2-cyclohexylethy-1-yl, tetrahydrofuranyl and the like.
The present invention relates to a method transforming a phenol I into a mixture of a p-bromophenol II and a p-benzoquinone III 
wherein, in each of structures I, II and III, R1 is independently at each occurrence a halogen, C1-C20 alkyl group, C4-C20 cycloalkyl group, or a C4-C20 aryl group, and n is an integer from 0 to 4.
In one embodiment of the present invention a product mixture comprising p-bromophenol II and benzoquinone III is subjected to a separation step to provide p-bromophenol II and p-benzoquinone III in purified form. A significant advantage of the method of the present invention over known methods is that it provides the intermediate p-benzoquinone III which may be transformed under very mildly reducing conditions to hydroquinone derivative IV 
wherein R1 and n are defined as in structures I-III. Purified p-bromophenol II may be transformed via reductive coupling into 4,4xe2x80x2-dihydroxybiphenyl derivative V 
wherein R1 and n are defined as in structures I-III.
In one embodiment of the mixture comprising p-bromophenol II and p-benzoquinone III is subjected to mild reduction of the p-benzoquinone component to hydroquinone IV prior to separation from p-bromophenol II. Reduction methods for transforming benzoquinones into the corresponding hydroquinones are well known in the art and include catalytic reduction using hydrogen and a noble metal catalyst, for example reduction with hydrogen gas using palladium on charcoal as the catalyst at ambient temperature and pressure.
Physical methods which may be used to separate a mixture comprising p-bromophenol II and p-benzoquinone III into purified forms of II and III include fractionation by vacuum distillation, steam distillation, reactive distillation, sublimation, crystallization, zone refining and like techniques, or a combination of one or more of such techniques. Typically, fractional distillation and crystallization are preferred. In one embodiment, the mixture of p-bromophenol II and p-benzoquinone III prepared by the method of the present invention is separated by fractional distillation and recrystallization into purified forms of p-bromophenol II and p-benzoquinone III having purities in excess of 90 percent, preferably in excess of 95 percent, and still more preferably in excess of 98 percent. In one embodiment a mixture comprising 4-bromophenol (CAS No. 106-41-2) and 1,4-benzoquinone (CAS No. 105-51-4) prepared from phenol using the method of the present invention, is separated by fractional distillation and recrystallization of the resultant distillation fractions to provide purified 4-bromophenol and 1,4-benzoquinone having purities in excess of 90 percent.
The method of the present invention is carried out by first contacting a hydroxy aromatic compound with hydrogen bromide (HBr), a source of copper, an organic solvent, and oxygen gas at a temperature in a range between about 20xc2x0 C. and about 250xc2x0 C., preferably between about 30xc2x0 C. and about 150xc2x0 C., and even more preferably between about 40xc2x0 C. and about 100xc2x0 C. The amount of hydrogen bromide plays a key role in producing high levels of benzoquinone product. The amount of hydrogen bromide should be in a range corresponding to between about 0.01 and about 0.2 moles, preferably between about 0.02 and about 0.15 moles, and still more preferably between about 0.04 and about 0.1 moles of hydrogen bromide per mole of said hydroxyaromatic compound.
Typically, the hydroxyaromatic compound is a phenol bearing no substituent in the position para to the phenolic OH group, for example structure I 
wherein R1 is independently at each occurrence a halogen, C1-C20 alkyl group, C4-C20 cycloalkyl group, or a C4-C20 aryl group, and n is an integer from 0 to 4.
Examples of phenols having structure I include phenol, o-cresol, m-cresol; 2,6-xylenol; 2,5-xylenol; 2,3,5-xylenol; 2,3,6-xylenol; 2-ethylphenol, 2-propylphenol, 2-tert-butylphenol, 2-phenylphenol, and the like. Typically, phenol and o-cresol are preferred.
The method of the present invention requires only catalytic amounts of copper but typically involves the use of a substantial stoichiometric excess of oxygen. Typically, the hydroxyaromatic compound is reacted in the presence of aqueous HBr, a copper species, an organic solvent, and oxygen in a reaction vessel equipped for intimate mixing of the reactants and operation at pressures higher than atmospheric pressure. In one embodiment, the reactants other than oxygen are first charged to a batch reaction vessel and stirring is begun. Subsequently a stoichiometric excess of oxygen gas is introduced into the reaction vessel to provide a partial pressure of oxygen over the reaction mixture of from about 50 pounds per square inch (psi) to about 2000 psi, preferably from 50 psi to about 1500 psi and still more preferably from about 100 psi to about 500 psi. In an alternate embodiment, the reactants are introduced into a flow reactor, for example a continuous stirred tank reactor, agitated and exposed to oxygen under conditions corresponding to partial pressure of oxygen over the reaction mixture of from about 50 pounds per square inch (psi) to about 2000 psi, preferably from 50 psi to about 1500 psi and still more preferably from about 100 psi to about 500 psi. In almost any embodiment of the invention, the oxygen may be used advantageously in the form of pure oxygen, air, a synthetic mixture of oxygen and one or more other gases (for example a synthetic mixture of oxygen and nitrogen), or any other convenient source of oxygen which may be used as the stoichiometric oxidant. Reaction vessels for use according to the method of include stirred tank reactors, continuous stirred tank reactors and the like.
As mentioned, the source of copper employed is present in a catalytic amount and can be any copper source capable of producing copper ions under the reaction conditions. Thus, even elemental copper or a mixture of copper compounds may be used according to the method of the invention. It has been found most convenient, however, to employ a catalytic amount of copper in the form of a single soluble copper compound, such as a copper halide. The initial oxidation state of the copper compound employed does not appear to be critical, so that cuprous halides and cupric halides may be employed with reasonable interchangeability. Typically the source of copper will be selected from the group consisting of cuprous chloride, cuprous bromide, cuprous iodide, cupric chloride, cupric bromide, and cupric iodide. Owing to their greater overall stability cupric halides are typically preferred.
Typically, the source of copper employed is used in an amount sufficient to provide a concentration of copper ion in the reaction mixture in an amount corresponding to between about 0.001 and about 0.200 moles, preferably between about 0.01 and about 0.1 moles, and still more preferably between about 0.03 and about 0.07 moles of copper ion per mole of hydroxyaromatic compound.
The method of the present invention may be advantageously carried out in the presence of an organic solvent which may be a pure solvent, or a mixture of solvents. Typically it is preferred that the organic solvent comprise an organic nitrile solvent, for example acetonitrile. Organic nitrile solvents are illustrated by acetonitrile, propionitrile, butyronitrile, isopropylnitrile, benzonitrile, and mixtures thereof. In some embodiments an organic nitrile solvent is used as a mixture with one or more solvents selected from the group consisting of aliphatic ethers, aromatic ethers, aliphatic alcohols, aromatic alcohols, ketones, halogenated alkanes, halogenated aromatics, amides, aliphatic hydrocarbons, and aromatic hydrocarbons. Typically, the organic solvent is present in an amount corresponding to between about 0.01 and about 1.0 liters, preferably between about 0.01 and about 0.5 liters, and sill more preferably between about 0.1 and about 0.4 liters of organic solvent per mole of hydroxyaromatic compound employed.
When the source of hydrogen bromide is concentrated hydrobromic acid (i.e. a mixture of hydrogen bromide and water comprising about 48 percent by weight HBr and about 52 percent by weight water) it has been found beneficial in some instances to add additional water to the reaction mixture. Thus, when concentrated hydrobromic acid is employed, water is added to the reaction mixture in an amount corresponding to about 0.4 moles to about 5 moles, preferably from about 1 mole to about 5 moles, and still more preferably from about 1 mole to about 3 moles of water per mole of hydroxyaromatic compound.
In one embodiment, phenol is reacted under the conditions of the present invention to provide a mixture of 4-bromophenol with 1,4-benzoquinone. This embodiment of the present invention is practiced by contacting in a reaction mixture at a temperature in a range between about 20xc2x0 C. and about 150xc2x0 C., phenol with: hydrogen bromide, at least one source of copper selected from the group consisting of cupric bromide or cuprous bromide, acetonitrile; and oxygen gas. The hydrogen bromide is present in an amount corresponding to less than 0.2 moles of hydrogen bromide per mole of phenol. The source of copper is present in an amount corresponding to between about 0.01 and about 0.20 moles of copper per mole of phenol. The acetonitrile is present in an amount corresponding to from about 0.01 to about 1.0 liters of acetonitrile per mole of phenol, and the xe2x80x9ccontactingxe2x80x9d is carried out under a partial pressure of oxygen of from about 50 to about 2000 pounds per square inch. The product obtained is a mixture comprising 4-bromophenol and 1,4-benzoquinone.
In alternate embodiment, o-cresol is reacted under the conditions of the present invention to provide a mixture of 4-bromo-2-methylphenol with methyl-1,4-benzoquinone. This embodiment of the present invention is practiced by contacting in a reaction mixture at a temperature in a range between about 20xc2x0 C. and about 150xc2x0 C., o-cresol with: hydrogen bromide, at least one source of copper selected from the group consisting of cupric bromide or cuprous bromide, acetonitrile; and oxygen gas. The hydrogen bromide is present in an amount corresponding to less than 0.2 moles of hydrogen bromide per mole of o-cresol. The source of copper is present in an amount corresponding to between about 0.01 and about 0.20 moles of copper per mole of o-cresol. The acetonitrile is present in an amount corresponding to from about 0.01 to about 1.0 liters of acetonitrile per mole of phenol, and the xe2x80x9ccontactingxe2x80x9d is carried out under a partial pressure of oxygen of from about 50 to about 2000 pounds per square inch. The product obtained is a mixture comprising 4-bromo-2-methylphenol and methyl-1,4-benzoquinone.
Typically the percentage of a phenolic compound having structure I, converted to p-bromophenol II and p-benzoquinone III is at least 20 percent, preferably at least 25 percent, and still more preferably at least 30 percent. Typically, the selectivity for the production of p-bromophenol II is in a range between about 20 and about 80 percent. This means that for every mole of phenolic compound I converted to products, between about 0.2 and about 0.8 moles of a p-bromophenol II is produced. Correspondingly, the selectivity for the production of p-benzoquinone III is in a range between about 80 and about 20 percent. Other products, for example 2-bromophenols, may be formed during the reaction such that the sum of the selectivities for the p-bromophenol II and the p-benzoquinone III need not be 100%.